Deterioration and loss of metal due to corrosion generally increases at elevated temperatures. For example, the oxidation rate of titanium, iron, nickel, zinc, and the like, and refractory metals such as molybdenum, tungsten, niobium and tantalum is a primary concern at high temperatures where a rapid reaction between the metal and atmospheric oxygen occurs. In addition to loss of material due to oxidation, oxygen or other gaseous contamination often occurs by the diffusion of a gaseous species into a metal section. The formation of oxide layers on metal surfaces may affect the structural integrity of a metal section and decrease the capacity of a metal section to be bonded to another surface. Similarly, unwanted diffusion of a gas into a metal surface may produce a decrease in ductility. It is known that other unwanted metal degradation may also occur at elevated temperatures.
In order to reduce unwanted corrosion of metal sections, numerous corrosion-resistant alloys have been developed such as titanium alloys. However, even corrosion-resistant alloys may oxidize at an unacceptable rate during high-temperature processing. As will be appreciated by those skilled in the art, most metals are subjected to hot working at some point in the forming process. The need for elevated temperatures during metal processing and the resultant increase in metal degradation has produced a number of prior art techniques to eliminate corrosive atmospheres from the environment of the metal during high-temperature processing. For example, hot working in large vacuum chambers or in inert gas environments is a common technique. However, the costly manufacturing facilities which are required in these processes add additional expense to the final product. In many applications, an oxide layer is removed from a metal section by machining or the like.
Numerous protective coatings have also been devised by which a highly corrosive resistant barrier is created on a metal surface. The most commonly used metallic coatings include tin, zinc, lead-tin alloys, nickel, chromium, cadmium, cooper, aluminum, bronze, brass, lead, iron and steel. These metallic coatings may be applied to a metal section using a variety of techniques such as hot dip processes where the article to be coated is immersed in a molten bath of the protective metal, by metal cementation where the protective metal is alloyed into the surface layer of the part, and by metal spraying. In metal spraying, the protective metal is heated and atomized while being propelled at a high velocity to the surface to be coated. As the molten particles impact the surface, they adhere firmly, providing a thin coating against corrosion.
Another widely used method of applying a protective coating to a metal surface is known as metal cladding. In metal cladding, a metal core having poor corrosion resistance is surrounded by a corrosion-resistant metal to from a layered product. The cladding may be formed by casting or by electrolytic deposition of the protective coating on the core. Additionally, a metal section may be placed between two sheets of a corrosion resistance metal, such as a section of flat steel placed between two sheets of aluminum. The assembly is then cold rolled to form a tri-laminate structure. Other cladding techniques such as fusion welding are also known. The clad article may then be further worked by extrusion, hot rolling, hot compaction, or other metal working techniques. In addition, it is known to apply protective coatings by other techniques such as cathode sputtering and evaporation/condensation deposition techniques. In many instances, where a protective coating is used only to encapsulate a metal section to prevent oxidation during processing, the encapsulant layer must then be removed either chemically or by various machining techniques.
In a number of applications, for example in the aerospace industry, dense, ductile metallic foils are often utilized. Although these foils may have good corrosion resistance at ambient temperatures and in the vacuum of space, they may undergo an unacceptable level of oxidation at elevated temperatures. In the past, these foils have been manufactured using complicated and costly vacuum evaporation processes whereby a metal-bearing coating material is vaporized within a vacuum. A portion of the metallic content of the vaporized coating material is then condensed on a substrate. Metallic foils manufactured by flame spraying a molten metal on the surface of a substrate are also known. These methods typically employ a release agent on the substrate such as a fluoride salt to facilitate the removal or stripping of the foil from the surface of the substrate. Metal deposition techniques of this nature have been used both with static substrates and with moving substrates which pass through a deposition chamber or under a flame spray nozzle in a continuous fashion. Foils may also be prepared by the machining of cast articles or by hot rolling under vacuum.
In U.S. Pat. No. 2,997,784 to Petrovich et al., a method of making composite metal articles is described in which a release agent is placed between two metal slabs of cladding material. The base material to be cladded is then placed in juxtaposed relation with the non-coated surfaces of the cladding layers. The assembly is then welded around the edges and rolled to the desired thickness, whereby the base metal is pressure-bonded to the cladding. The marginal edges are then removed, and the two cladded slabs of base metal are separated. It is disclosed that calcium fluoride and other fluorides can be used as parting compounds which may be sprayed onto the cladding layers as an aqueous solution or slurry. It is also disclosed that the base metal can be applied to the cladding layers by placing the cladding layers between which the parting compound is disposed in a mold with the base metal being then cast in place around the cladding layers.
In U.S. Pat. No. 3,164,884 to Noble et al., a method for the multiple rolling of sheets is disclosed in which cover plates and side bars are assembled around inner plates separated by a separating compounds. The side bars are provided with vent holes and are arc welded along their outer edges to the cover plates and to each other. The separating compounds which are disclosed include aqueous mixtures of oxides, specifically chromium, magnesium and aluminum oxides. The vent holes permit gases in the sandwich to escape during heating and rolling.
As will be appreciated by those skilled in the art, the prior art techniques of fabricating thin sheets or foils all have considerable drawbacks which make them undesirable in terms of cost, production capacity, and quality control. Therefore, it would be desirable to provide a cost-effective method of producing thin metal sections such as foils which reduces or eliminates destructive oxidation during high-temperature processing. The present invention achieves this goal by providing a method by which reactive metals can be formed into thin sections in a hot working process which can be carried out in an unmodified atmosphere at ambient pressure and which does not require complicated machining or chemical stripping of an encapsulant.